Can scientists’ “theory of everything” really explain all the

weirdness the universe displays? / / / BY EDWARD WITTEN

Astronomers have wrapped up cosmic history The story begins early in the 20th century, in a neat package. Or so it might seem. Some when Albert Einstein drastically changed our 12 to 14 billion years ago, the universe came notions about space and time. In Einstein’s into existence — along with space and time theory, space and time, which in everyday themselves. A fraction of a microsecond later, experience seem completely different, were inflation set in, and for a brief period, the unified into a strange new concept that came cosmos expanded at an explosive rate. Within to be called space-time. His ideas have held a billion years, galaxies began to form with up well — almost everything else in our fun- the aid of dark matter, which still holds them damental description of physics has changed together. And now, a mysterious force known since Einstein’s day, but we still describe as dark energy seems to be taking over, accel- space-time using the concepts he introduced. erating the universe’s ongoing expansion. Yet many scientists suspect this is destined to Yet this picture just skims the surface. change and that new developments in the Scientists aim to dig deeper, to understand why understanding of space-time will need to things happened the way they did. What was emerge before we can attack the grand puz-

Updated from the June 2002 issue of Astronomy

the Big Bang, and how could time just begin? zles ahead. What caused cosmic inflation? And what, Actually, Einstein transformed our concepts exactly, are dark matter and dark energy? of space and time with two startling revela- Many scientists believe the answers to tions. The first upheaval came in 1905 with the these questions are tied up with some of the theory of special relativity, which explored the deepest unsolved problems in physics. A strange behavior of matter moving near the thirty-year-old framework known as string speed of light. The theory showed how clocks theory promises new insights and offers hope carried on a rapidly moving spaceship slow that answers to at least some of these puzzles down and the heartbeat of an astronaut almost may be on the horizon. stands still — as seen by an observer at rest.

Universe STRING THEORY suggests that particles in the universe are composed of loops of vibrating strings. Like a violin or piano string, one of these fundamental strings has many different harmonics. In string theory, harmonics correspond to different elementary particles. If string theory proves correct, electrons, photons, and neutrinos are different due to changes in the vibrations of the strings. ILLUSTRATION: ASTRONOMY: CHUCK BRAASCH

042 Origin and Fate of the Universe

on a string www.astronomy.com 043 Ten years later, when most scientists had barely recovered from ACCORDING TO EINSTEIN, gravity arises because massive objects, like thethe first shock, the second revolution arrived. In 1915, Einstein Sun, warp space-time, causing smaller objects like Earth to orbit them. ASTRONOMY: ROEN KELLYcompleted his greatest and most surprising achievement: the the-ory of gravity known as general relativity. According to general ics, as elaborated in the standard model, described small objectsrelativity, space-time is curved, and the curvature is created by such as atoms, molecules, and subatomic particles.matter. When planets travel in elliptical orbits around the Sun, for Physicists, however, are not satisfied having two different theo-example, they are merely seeking the most direct paths in the ries that work in two different realms. One reason is simply thatcurved geometry created by the Sun’s gravity. big objects ultimately are made out of little objects. The same Surprising predictions from special relativity ranged from an forces operate on both atoms and stars, for example, even thoughultimate speed limit — nothing can travel faster than the speed of gravity is more obvious for stars while electricity and magnetismlight — to the famous formula, E=mc2, that describes the equiva- dominate in atoms. So it must be possible to combine the stan-lence of mass and energy. General relativity, on the other hand, dard model and general relativity into a bigger, more completepredicted gravitational waves, black holes, the bending of light by theory that describes the behavior of both atoms and stars.the Sun, and the expansion of the universe. Moreover, the quest for unification has paid enormous divi- After Einstein, further discoveries changed almost everything dends in the past. Both the standard model and general relativityelse in our understanding of physics. Scientists discovered new were discovered, in large part, through efforts to unify earlier the-building blocks of matter and the surprising laws that govern ories. Unfortunately, direct attempts to express general relativitytheir behavior. But all the new phenomena occurred, and all the in quantum mechanical terms have led to a web of contradic-new particles were found, in the space-time arena that Einstein tions, basically because the nonlinear mathematics Einstein usedhad set forth. to describe the curvature of space-time clashes with the delicate In the 1920s, we learned that subatomic particles obey not requirements of quantum mechanics.Newton’s laws of motion but the weird and wonderful laws ofquantum mechanics, in which particles behave as waves and String theory to the rescue?Heisenberg’s uncertainty principle (you can know an electron’s Occasionally, when scientists face a big problem like this one,velocity or its position, but not both) gives everything a fuzziness someone disappears into an attic for seven years and emergesnearly impossible to describe in words. By the mid-1970s, quan- with an answer. That, more or less, is how Andrew Wiles provedtum theory was expanded into a theory of elementary particles — Fermat’s Last Theorem. In the case of quantum gravity, the “sci-the standard model of particle physics — which in its own realm entist in an attic” approach has never borne much fruit. Luckis every bit as successful as Einstein’s theory. played a role instead: Physicists who originally had quite a differ- By the 1970s, a clear division of labor existed in our under- ent goal in mind stumbled onto a promising approach.standing of physics. General relativity described large objects such This came about in the early 1970s with the development ofas the solar system, galaxies, and the universe. Quantum mechan- string theory. According to string theory, an elementary particle is not a point but a loop of vibrating string. Just like a violin orEdward Witten is a professor of physics at the Institute for Advanced Study piano string, one of these “fundamental strings” has many differ-in Princeton, New Jersey. ent harmonics or forms of vibration. For a piano string, the044 Origin and Fate of the Universe /// UNITING THE FOUR FUNDAMENTAL FORCES OF NATURE WITH STRING THEORY

GRAVITY describes the attractive ELECTROMAGNETISM describes THE STRONG NUCLEAR FORCE is THE WEAK NUCLEAR FORCE force of matter. It is the same how electricity and magnetic responsible for holding the nucle- explains beta decay and the force that holds planets and fields work. It also makes objects us of atoms together. Without this associated radioactivity. It also moons in their orbits and keeps solid. Once believed to be two force, protons would repel one describes how elementary our feet on the ground. It is the separate forces, it was discovered another so no elements other than particles can change into other weakest force of the four by many both could be described by a hydrogen — which has only one particles with different energies orders of magnitude. simple set of equations. proton — would be able to form. and masses.

ASTRONOMY: RICK JOHNSON

The equations that describe gravity, including Einstein’s theory of general being either zero or infinity. Neither answer is worse; describing an object relativity, predict the behavior of objects on macroscopic scales extremely bigger than the (finite) universe — or one that doesn’t exist — are both accurately. In the microscopic world, however, electromagnetism, the equally impossible. strong nuclear force, and the weak nuclear force dominate. Collectively, Here’s where string theory comes to the rescue. By adding seven they provide the foundation of quantum mechanics. Because gravity is so hidden dimensions to the familiar three and another for time, plus weak at small scales compared to the other forces, particle physicists don't antiparticles and a mirror set of particles called superparticles, the math even bother to account for it in their experiments. To illustrate this, imagine starts to make sense. The force of gravity is diluted because it permeates a single proton lying on the floor and another suspended one yard (meter) into one or more of the hidden dimensions. Dark matter and dark energy above it. The strong nuclear force is so powerful that the top proton’s attrac- also may invisibly shape our universe from these phantom dimensions. tion easily outmuscles the gravitational pull of the entire Earth. So how do we know if string theory is real or just a mathematical Why try to unite the four forces in a single theory? Why not simply abstraction? At this point, no one has devised an experiment that can use Einstein’s theory of general relativity to govern big things and quan- prove or disprove it. Critics say there will never be such an experiment. tum mechanics for little ones? Some concepts, such as the Big Bang or Proponents, however, see such beauty and symmetry in the equations — how black holes form, live in both domains. When we combine equations as nature has revealed so often in the past — that it would be tragic if of the four forces to describe these ideas, our answers usually end up some form of string theory was not real. — Tom Ford

harmonics consist of a basic note — such as middle C — and its Back in the early 1970s, one of the pioneers of string theory,higher overtones (one, two, or several octaves higher). The rich- Italian physicist Daniele Amati, characterized the theory as “partness of music comes from the interplay of higher harmonics. of 21st-century physics that fell by chance into the 20th century.”Music played with a tuning fork, which produces only a basic He meant that string theory had been invented by chance andnote, sounds harsh to the human ear. developed by a process of tinkering, without physicists really In string theory, different harmonics correspond to different ele- grasping what was behind it. Amati surmised that a true under-mentary particles. If string theory proves correct, all elementary standing of the foundation of this remarkably rich theory wouldparticles — electrons, photons, neutrinos, quarks, and the rest — have to await the 21st century.owe their existence to subtle differences in the vibrations of strings. Thirty years later, we have a firmer grip on many issues, yetThe theory offers a way to unite disparate particles because they there’s still much we don’t understand at all — including the foun-are, in essence, different manifestations of the same basic string. dations of string theory. On the other hand, with the 21st century How does this help us with gravity? In the early 1970s, calcula- only just begun, we have yet to fall behind Amati’s schedule!tions showed that one of the string’s vibrational forms had justthe right properties to be a graviton, the basic quantum unit of The fuzziness of space-timegravity. Curiously, like many of the most important discoveries in Perhaps the most basic thing we have learned about string theorystring theory, this one came about when a researcher made a is that it modifies the concepts of space-time that Einstein devel-technical calculation without realizing, at the time, the full impli- oped. This doesn’t come as a complete surprise: Einstein based hiscations of his work. theory of gravity on his ideas about space-time, so any theory that From little acorns grow mighty oaks. A quantum theory with modifies Einstein’s gravitational theory to reconcile it with quan-gravitons must, according to arguments that physicists have tum mechanics has to incorporate a new concept of space-time.known for years, incorporate the full structure of Einstein’s theo- String theory actually imparts a “fuzziness” to all our familiarry — at least in circumstances involving astronomical bodies notions of space and time, just as Heisenberg’s uncertainty princi-where general relativity successfully applies. (At the atomic level, ple imparts a basic fuzziness to classical ideas about the motion ofsuch a theory has to depart from Einstein’s, which doesn’t work particles. In ordinary quantum mechanics, interactions amongquantum mechanically.) elementary particles occur at definite points in space-time. In www.astronomy.com 045The beautyof string theory space

Time is added to form another dimension.

time Strings may be open ended or closed loops.

In string theory, elementary particles are not

points but vibrating strings. The frequency of thestring determines what type of particle it is.ASTRONOMY: ROEN KELLY

string theory, things are different: Strings can interact just as par- into our description of space-time. But how can we ever know ifticles do, but you cannot say quite when and where this occurs. supersymmetry is right? Even to a theoretical physicist, this kind of explanation raisesmore questions than it answers. String theory involves a concep- Discovering supersymmetrytual jump that’s large even compared with previous revolutions in In a world based on supersymmetry, when a particle moves inphysics. And there’s no telling when humans will succeed in cross- space, it also can vibrate in the new fermionic dimensions. Thising the chasm. new kind of vibration produces a cousin or “superpartner” for Nevertheless, we really do understand one aspect of how string every elementary particle that has the same electric charge buttheory changes our notions of space-time. This involves a key differs in other properties such as spin. Supersymmetric theoriespart of string theory called supersymmetry. Finding supersymme- make detailed predictions about how superpartners will behave.try offers cosmologists’ best and brightest hope of proving that To confirm supersymmetry, scientists would like to produce andstring theory has something to do with nature and is not just study the new supersymmetric particles. The crucial step is build-armchair theorizing. ing a particle accelerator that achieves high enough energies. In our everyday life, we measure space and time by numbers. At present, the highest-energy particle accelerator is theFor example, we say, “It is now 3 o’clock,” “We are 200 feet above Tevatron at Fermilab near Chicago. There, protons and antipro-sea level,” or “We live at 40° north latitude.” This idea of measur- tons collide with an energy nearly 2,000 times the rest energy ofing space and time by numbers is one bit of common sense that an individual proton. (The rest energy is given by Einstein’s well-Einstein preserved. In fact, in his day, quantities that could be known formula E=mc2.) Earlier in this decade, physicists capital-measured by numbers were all physicists knew about. ized on Tevatron’s unsurpassed energy in their discovery of the But quantum mechanics changed that. Particles were divided top quark, the heaviest known elementary particle. After a shut-into bosons (like light waves) and fermions (like electrons or neutri- down of several years, the Tevatron resumed operation in 2001nos). Quantities like space, time, and electric field that can be meas- with even more intense particle beams.ured by numbers are “bosonic.” Quantum mechanics also In 2007, the available energies will make a quantum jump whenintroduced a new kind of “fermionic” variable that cannot be meas- the European Laboratory for Particle Physics, or CERN (locatedured by ordinary numbers. Fermionic variables are infinitesimal and near Geneva, Switzerland) turns on the Large Hadron Collider, orinherently quantum mechanical, and as such are hard to visualize. LHC. The LHC should reach energies 15,000 times the proton rest According to the idea of supersymmetry, in addition to the energy. The LHC is a multi-billion dollar international project,ordinary, familiar dimensions — the three spatial dimensions funded mainly by European countries with substantial contribu-plus time — space-time also has infinitesimal or fermionic tions from the United States, Japan, and other countries.dimensions. If supersymmetry can be confirmed in nature, this If our hunches prove correct, there’s an excellent chance thatwill begin the process of incorporating quantum mechanical ideas supersymmetry lies within reach of the LHC, and maybe even of046 Origin and Fate of the Universethe Tevatron. Many physicists suspect the LHC will producesupersymmetric particles at a huge rate. If that happens, elemen-tary particle physics will enter a completely new era, with theexperimental study of phenomena derived from the quantum timestructure of space-time. The next step would be to study super-symmetric particles in detail and extract crucial clues that couldhelp us understand string theory or whatever deeper theoryunderlies supersymmetry. The Tevatron and the LHC accelerate protons — and, in thecase of the Tevatron, antiprotons also. Proton accelerators affordthe easiest way of reaching the highest possible energy because pro-tons can be accelerated much more easily than other particles. P Unfortunately, proton accelerators have a drawback. They typ- timeically produce dozens of uninteresting particles along with theparticles of interest. The supersymmetric world is far too compli- Qcated to be explored fully at a proton accelerator. For accuratemeasurements, we need a different kind of machine — one thataccelerates electrons and their antiparticles, positrons. WHEN A SINGLE ELEMENTARY The highest-energy electron accelerators built so far have been at PARTICLE breaks in two (inset), itCERN and at the Stanford Linear Accelerator Center in California. occurs at a definite moment inThese machines have carried out the most precise and complete space-time. When a string breakstests of the standard model of particle physics. In the last decade, the into two strings (right), different observers will disagree aboutUnited States, Japan, and Germany have devoted intensive research when and where this occurred. Aand development toward a higher-energy electron accelerator relativistic observer who consid-known as the Linear Collider, which could reach the energy level ers the dotted line to be a surfaceneeded to study supersymmetric particles. of constant time believes the The yet-to-be-approved Linear Collider, like the LHC, will be a string broke at the space-timemulti-billion dollar project that can be built only with extensive point P while another observer who considers the dashed line to be a sur-international cooperation — perhaps encouraged by the curiosity face of constant time believes the string broke at Q. ASTRONOMY: ROEN KELLYof the public in the countries involved. energy). The vacuum energy is a problem that involves both quan-The astronomy connection tum mechanics, because this energy comes from quantum fluctua-What about those astronomical mysteries we started with? None tions, and gravity, because gravity is the only force in nature thatof the problems yet has any definitive solution, but physicists sus- “sees” the energy of the vacuum. Because string theory is the onlypect solutions are linked to the exploration of supersymmetry, framework we have for understanding quantum gravity, the vacuumstring theory, and the quantum nature of space-time. energy poses a problem for string theorists that remains to be solved. First, although other possibilities exist, many physicists think As for inflation, scientists believe it occurred in the early uni-galactic dark matter is a cloud of supersymmetric particles gravita- verse at a temperature far above the energy attainable with parti-tionally bound to a galaxy. Calculations show that such a cloud cle accelerators and relatively close to the energy at whichwould have just about the right properties. If this supposition quantum gravity becomes important. We do not yet have a con-proves correct, dark matter will be detected in the next decade. vincing model of how and why inflation transpired because ourSpecial underground detectors can spot the rare interactions of dark current models of particle physics are not adequate at the enor-matter particles passing through Earth. The instruments lie deep mous energy levels of inflation. Understanding inflation requiresunderground, often in mines, to screen them from cosmic rays. a much better grasp of particle physics than we now have, and Detecting dark matter would be a milestone in astronomy, but possibly a full knowledge of string theory and quantum gravity.not the whole story. Underground detectors would measure the Finally, what was the Big Bang all about, and how could thereproduct of the density of dark matter particles times the interac- have been a beginning to space and time? This question certainlytion rate, but that’s only a partial solution. To learn how much involves quantum gravity, because quantum mechanics and gen-dark matter of this type exists, scientists would need to measure eral relativity were both important near the Big Bang. That cre-the interaction rate by producing dark matter particles in acceler- ates another grand challenge for string theorists, even though weators and observing their properties. Thus, the LHC and the do not seem close to an answer. A plausible guess springs fromLinear Collider, together with supersymmetry, might be key to the way quantum gravity and string theory impart a fuzziness tounderstanding dark matter. our concepts of space-time. Under ordinary conditions, time The dark energy problem is a more difficult challenge. One of the seems like a well-defined notion, but as you get closer to the Bigmost dramatic discoveries in astronomy and physics in recent years is Bang, quantum mechanical and stringy fuzziness become morethat the expansion of the universe seems to be accelerating. This significant. The very notion of time may lose its meaning whenpoints to a tiny but positive energy density of the vacuum (or possi- one gets back to the beginning — and that, quite likely, will provebly a more complicated scenario involving another form of dark to be a key to understanding what the Big Bang really was. X www.astronomy.com 047